Ink composition for offset printing is typically composed of a colorant represented by pigment; a binder resin represented by rosin-modified phenol resin and rosin-modified maleic resin; a drying oil such as tung oil, linseed oil and soybean oil, used for forming a coated film; a dryer used as a catalyst for promoting oxidative polymerization (dry curing using aerial in the air); and a petroleum solvent such as mineral oil. The ink composition may be optionally added with a variety of additives, aiming at improving suitability to printing related to printing presses.
An exemplary method of printing using an ink composition for offset printing of books, posters, calendars and so forth will now be explained. FIG. 1 is a schematic drawing for explaining an exemplary method of printing using the ink composition for offset printing, where reference numeral 1 stands for a petroleum solvent, 2 for a printing pacer, 3 for oxygen, 4 for a colorant, 5 for a binder resin, and 6 for a drying oil. Now as illustrated in FIG. 1, printing follows processes of allowing the petroleum solvent 1 contained in the ink composition for offset printing to permeate into the printing paper 2, allowing the residual components which remain on the surface of the printing paper to be oxidized by aerial in the air 3, polymerized and dried, to thereby form a solid coating on the surface of the printing paper. In FIG. 1, (1) illustrates a state immediately after printing, (2) illustrates a state in the middle way of set drying (which means permeation of solvent into paper, also referred to as permeation drying), and (3) illustrates a state after completion of oxidative polymerization and drying. Immediately after the printing (1), an ink composition layer applied to the surface of the printed matter does not dry. In the step of set drying (2), the solvent in the ink composition permeates into the printing paper, and thereby the ink composition increases the viscosity. Although the ink composition layer becomes somewhat durable against pressurizing owing to the thus increased viscosity, the set drying (2) is not yet a step of complete drying. Although the set drying (2) and the oxidative polymerization and drying (3) can proceed concurrently, the set drying completes within a time scale of several minutes, meanwhile the oxidative polymerization and drying needs several hours. It takes only several seconds for the printing paper to be input and output to and from a printing press. The printing matter is, therefore, output from, the printing press before being fully processed by the steps of set drying (2) and oxidative polymerization and drying (3). As a consequence, the printed matters when stacked (or column-stacked) after the printing may adhere to each other. In particular, adhesion of the ink, before being thoroughly set-dried, onto the back face of the printed matter is called “offset”. Adhesion of the ink onto the back face of the column-stacked printed matters, due to delayed drying of the ink and pressurizing on the paper, is called “sticking”. Meanwhile, adhesion of the printed faces of the column-stacked printed matters so as to form a block, due to re-softening of the ink even having once been completely dried, is called “blocking”.
In these years, an ink composition having a reduced content of, or even no, petroleum solvent has been attracting public attention, from the environmental, or safety and hygienic points of view, and is becoming more popularly used. As one example of such ink composition, efforts have been made to replace a part of, or all of, petroleum solvent with a drying oil such as soybean oil. The ink composition, however, often tends to delay elimination of the petroleum solvent contained therein, as the content of drying oil such as soybean oil increases, and to delay permeation into the printing paper. This unfortunately worsens offset or sticking, characterized by adhesion of ink onto the back face of printing paper.
A popular way to avoid offset and blocking is to scatter a powder, such as corn starch, over the printing paper immediately after printing. Such scattering of powder, however, degrades the work environment due to dusting of the powder per se, shortens the cleaning cycles, and lowers the production efficiency. It also causes so-called “scaling”, which is an accidental fall of the powder, having been accumulated on a delivery portion inside the printing press, onto the printed matter during printing. The powder not only degrades the quality of printed product, but also inhibits PP lamination in the post process, which is a step of laminating a polypropylene sheet onto the printed matter. It has also been pointed out that the fine powder entrained, inside the printing press may heavily wear the mechanical parts, may induce failure, and may shorten the service life of the press.
As solutions to solve this problem, there have been proposed methods of incorporating a predetermined amount of a polymer having a solubility parameter of 19 (MPa)1/2 or smaller, and compatible with the solvent component, into the ink composition for offset printing (Patent Literature 1, Patent Literature 2); a method of incorporating a solvent composed of a plant oil component and liquid paraffin into the ink composition for offset printing (Patent Literature 3); and a method of incorporating a composition for planographic liquid which contains an acrylic copolymer having a glass transition temperature of 20° C. or higher and having a weight-average molecular weight of 30,000 to 500,000, and an aliphatic hydrocarbon (Patent Literature 4).
Also from the environmental point of view, there has been a diversified trend of printing paper, including non-wood pulp paper made of fiber of kenaf or rice straw, and decorative paper called fancy paper, rather than recycled paper recycled from used paper or wood pulp. This sort of printing paper, however, allows therein only a slow permeation of the solvent contained in the ink composition, making the paper less dryable, and more likely to cause problems such as offset and sticking.
A UV curable ink composition has been proposed to solve the problem (Patent Literature 5). The UV curable ink composition can dry and cure under UV irradiation within a very short time, and will have high friction resistance, anti-blocking performance, and anti-offset performance, without needing the powder spray.
The UV curable ink composition, however, needs an additional UV irradiation equipment, and is not economical. The composition also needs a special UV curable monomer, oligomer and photo-polymerization initiator, making the ink composition expensive. The ink composition is not preferable also from the viewpoint of energy consumption, since it requires not only electric power for operating the printing press, but also electric power fed to a UV lamp used for irradiating UV light.
Also there is proposed a method of using an OP varnish typically containing coarse processed starch or resin (Patent Literature 6). The method is, however, disadvantageous not only because the OP varnish is clear and the offset is not recognizable if occurred, on the printing paper, and also because the offset may occur on the printing paper on which the ink can set-dry only slowly, in the process of stacking the printed matters during printing.
Patent Literature 7 describes mixing of an anti-blocking powder into a printing ink.
Patent Literature 8 discloses an ink composition for dry planographic printing, which contains a homopolymer of a (meth) acrylate ester produced from acrylic acid or methacrylic acid, reacted with a monohydric alcohol having a number-average molecular weight of 5 to 40. A polymer of 2-ethylhexyl acrylate described in Patent Literature 8 has a glass transition temperature of 0° C. or below.
Patent Literature 9 describes a printing ink which contains a copolymer composed of isobornyl methacrylate and ethylenic unsaturated monomer, having a weight-average molecular weight of 4,000 to 12,000, and a glass transition temperature of 50 to 120° C.
Patent Literature 10 discloses a resin composition for ink, which contains an acrylic copolymer containing 4-t-butylcyclohexyl (meth) acrylate monomer and a vinyl-based monomer having a functional group. Patent Literatures 9 and 10 employ a printing system capable of forming an ink composition film without using a drying oil.
In the printing market today, requests for shorter delivery are increasing acceleration. In such circumstances, there are strong needs for improving the productivity without any new facility investment, improving the environmental hygiene which may be degraded by the scattered powder around the printing press, and elongating the mechanical service life of the printing press and the peripheral equipment.